1. Field of the Invention
The present invention relates to a control apparatus for an information storage and retrieval system, such as an optical recording and retrieval system, having a signal recording and reading unit for recording and reading signals on a recording medium, wherein a moving part such as an objective lens of the unit is controlled by feedback using signals read from the recording medium.
2. Description of the Prior Art
There has been known such a control apparatus for an information system as disclosed in Japanese Patent No. 1092602, and there has also been proposed an optical disk recording and reproduction system of a tracking servo type, as shown in FIG. 1.
Included in the conventional apparatus shown in FIG. 1 are a light source 1 of, for example, semiconductor laser, a collimator lens 2 for producing a parallel light beam from the light emitted from the light source 1, a prism 3 for unifying the divergence angles in all directions of the beam coming from the light source 1, a condensing objective lens 4 for converging the parallel beam, a disk-type recording medium (will be termed simply "disk") 5, a guide track 6 formed in advance on the surface of the disk 5, a disk drive motor 7 for turning the disk 5, an objective lens drive unit 8, constructed similarly to the voice coil of a loud speaker, for moving the objective lens 4 in a direction substantially perpendicular to the groove of guide track 6 so that a small beam spot produced by the objective lens 4 is kept focused at the center of the guide track 6 on the turning disk 5, a quarter-wave plate 9 for shifting the phase of the beam passing therethrough by 1/4 wavelength, i.e., a total of 1/2 wavelength for the return beam, a beam splitter 10 for bending the optical path of the 1/2 wavelength shifted beam by approximately 90.degree., a split photo-sensor 11 for transducing the beam coming from the beam splitter 10 into electrical signals, an operational circuit 12 for obtaining the difference of the outputs of the split photo-sensor 11, an amplifier 13 for amplifying the output of the operational circuit 12, a phase compensation circuit 14 for compensating the phase log of the objective lens drive unit 8, a drive circuit 15 for activating the objective lens drive unit 8, an operational circuit 16 for summing the outputs of the split photo-sensor 11, an amplifier 17 for amplifying the output of the operational circuit 16, a division circuit 18 for dividing the output A of the amplifier 13 by the output B of the amplifier 17, and a drive circuit 19 for modulating the light source 1.
In the foregoing arrangement, the light source 1, collimator lens 2, prism 3, objective lens 4, objective lens drive unit 8, quarter-wave plate 9, beam splitter 10, and split photo-sensor 11, in combination, constitute a signal recording and retrieval system for recording and reading signals on the recording medium. The objective lens 4 is a moving part of the system.
The operation of the foregoing conventional apparatus will be described. The light emitted by the light source 1 is formed into a parallel beam by the collimator lens 2, and after having been shaped by the triangular prism 3 it is propagated through the beam splitter 10 to the quarter-wave plate 9. The beam has its phase shifted by 1/4 wavelength by the quarter-wave plate 9, and it is converged by the objective lens 4 onto the guide track 6 on the disk 5.
In recording information on the disk 5, the drive circuit 19 operates on the light source 1 in response to the information signal to be recorded to increase the energy density of the beam spot in the groove of the guide track 6 sufficient to change the reflectivity of the recording medium in accordance with the information signal. For example, when the binary information signal as shown by (A) in FIG. 2 is to be recorded, the output of the light source 1 is increased in response to the "1"s level of the signal so as to vaporize the recording material of the guide track 6 in that portion thereby to lower significantly its reflectivity in contrast with unaffected portions corresponding to "0"s level of the recording signal.
In the reproducing operation, for example, for the information signal as shown by (A) in FIG. 2 recorded on the guide track 6, the output of the light source 1 is maintained at a level lower than that for recording on the recording medium as mentioned above. The beam reflected on the guide track 6 is collected by the objective lens 4, and after having been further phase shifted by 1/4 wavelength by the quarter-wave plate 9 it is bent by approximately 90.degree. by the beam splitter 10 and received by the split photo-sensor 11. The guide track 6 has a convex or concave structure of around 1/8 wavelength relative to its periphery, causing diffraction in the reflected beam, and therefore a displacement of the converged beam spot with respect to the guide track 6 causes anisotropy in the reflected beam. Accordingly, by taking the difference of the outputs of the split photo-sensor 11, the error signal used for the tracking control of the objective lens 4 can be obtained. Furthermore, by taking the sum of the outputs of the split photo-sensor 11, the recording information signal (the output of the amplifier 17 in recording mode) as shown in by (B) in FIG. 2 is obtained for the input information signal as shown by (A) in FIG. 2, and also the reproduced information signal (the output of the amplifier 17 in reproduction mode) as shown by (C) in FIG. 2 is obtained for the same input information signal recorded on the disk 5, provided that recording takes place in such a mode of lowering the reflectivity of the recording material in response to the "1"s input signal level.
The error signal is amplified by the amplifier 13, then divided by the division circuit 18 by the signal amplified by the amplifier 17. The output of the division circuit 18 is the corrective error signal, which is phase compensated by the compensation circuit 14 and fed to the drive circuit 15 for the lens drive unit 8 thereby to control the objective lens 4 on a feedback basis so that the beam spot converged by the objective lens 4 is positioned at the center of the guide track 6. The reason for the use of the output of the division circuit 18 as the input of feedback control of the objective lens 4 is to cancel the variation in the loop gain of tracking servo system due to the variation in the light intensity which amounts 5:1 to 10:1 between recording and reproduction modes, the variation in the reflectivity of the recording medium during reproduction mode and the variation in the transmittance of the optical system.
On the other hand, different from the information signal, the code signal is generally recorded as convexes in the recording medium, and therefore even if the region of the code signal and the region of the information signal have the same amount of incident light, the magnitude of the error signal and reflected light intensity will differ. Furthermore, since the region of the code signal is smaller than that of the information signal and the servo gain is adjusted by making reference to the region of the information signal, the foregoing conventional feedback control apparatus is apt to cause a disturbance of operation, such as overshooting, when the beam spot is located in the region of the code signal. This improper operation will further be explained by taking one example shown in FIG. 3 for the case where the code signal is recorded in advance in the form of concaves with a depth equal to that of the guide track 6 on the disk 5.
Assuming that the disk 5 has a record of information as shown by (A-1) as a disk plan view and (A-2) as a disk cross-sectional view in FIG. 3 (the information signal is recorded in the form of pits on the guide track 6), the operational circuit 16 provides the output as shown by (B) in FIG. 3, which is reduced to the waveform shown by (C) in FIG. 3 due to the frequency response of the amplifier 17 as provided for the purpose of stabilizing the operation. On the other hand, the convex portions (in a sense of relativity) of the code signal and the pit portions of the information signal do not cause anisotropy by interference for the reflected beam, causing the operational circuit 12 to have zero output signal level Vo in principle, but actually an offset is created as shown in the waveform of (D) in FIG. 3. Then, through the same process as for the waveform of (B), the amplifier 13 provides the waveform shown by (E) in FIG. 3. Consequently, the division circuit 18 produces the output as shown by (F) in FIG. 3, creating a disturbance of operation at a transition of beam spot from the code signal to the information signal, and vice versa.
On this account in order to correct the variation in the loop gain and offset caused by the variation of reflected light intensity which amounts about 5:1 to 10:1 between recording and reproduction mode and also the variation of reflectivity of the recording medium which amounts about 1:2 during reproduction mode, the foregoing conventional apparatus needs a division circuit of high reliability. Such an apparatus is not only costly by itself, but also disadvantageous in some cases that the correction through the division operation is not sufficient since the detected signal for the information signal and the error signal are not completely proportional. This matter holds true in the focusing servo system and jitter servo system which have exactly the same arrangement as described above.
A control apparatus for information system such as an information recording and/or reproduction system in which the servo loop gain can be maintained constant without using the division circuit has been proposed by the inventors of the present invention and coinventors, Y. Tamura and I. Watanabe, and is described in Japanese Patent Publication (Japanese Patent Application Laid-open No. 58-32093, filed on Feb. 28, 1983). This information system is arranged such that the outputs of the split photo-sensor are fed to an operational circuit for producing the difference of these outputs and an operational circuit for producing the sum of these outputs, and the error signal provided by the former operational circuit is sampled and held by a sample holding circuit in a region where the waveform of the output of the latter operational circuit after being shaped by a waveform shaper is higher than the preset level, thereby controlling the position of the objective lens on a feedback basis using the error signal provided by the sample holding circuit.
However, this arrangement needs a high-speed sample holding circuit, which is less expensive than a division circuit, but more expensive as compared with the average cost of electronic circuits, and furthermore if the output level of the split photo-sensor is small, the switching noise accompanied by the sample-hold operation can not be ignored.
Generally, the address signal for identifying the information signal and the sensor signal for segmenting a track are recorded on the recording medium in a different or similar way of recording before or during the recording operation of the information signal, and these signals affect the error signal to change the gain, resulting possibly in the unstable control operation. Therefore, this prior art system has problems to be solved before being put into practice.